1. Field of the Invention
The present invention relates to an apparatus and method for maintaining non-volatility in a ferroelectric random access memory (FRAM) which enables preservation of recorded information during power failure.
2. Description of Related Art
Generally, a FRAM is classified into two types: 1T-1C having a one-transistor, one-capacitor structure, and 2T-2C having a two-transistor, two-capacitor structure depending on the structure of a unit memory cell. Even though the structure of each FRAM is different, a ferroelectric capacitor as a constituent thereof operates under the same principle. Also, there is an advantage of non-volatility of recorded information in the FRAM since the polarization state of a ferroelectric material is maintained during a power failure. The polarization state changes according to the direction of an externally applied electrical field, and the change in polarity is shown in FIG. 1. That is, a ferroelectric material has two polarization states, +Pr and -Pr, during a power failure. Also, the two polarization states are changed according to the polarity of the voltage applied to a capacitor as shown in FIGS. 2 and 3. Thus, the FRAM operates in a different manner than other memory devices. As the most significant difference, the FRAM applies a read pulse to an electrode of the capacitors to distinguish the polarization states of the capacitors by polarity. In the FRAM adopting the above method of operation, the polarization state may be inverted after a read is completed.
The change in the polarization state of the ferroelectric material according to the polarity of the voltage will be described in detail as follows.
The ferroelectric material has two polarization states, +Pr and -Pr, during a power failure according to the polarity of the voltage provided just prior to the polarization, as shown in FIG. 1. Also, the degree of polarization representing the polarization state varies depending on the polarity of the voltage just previously applied to the capacitor and is represented by a hysteresis curve as shown in FIGS. 2 and 3. For example, the ferroelectric material is in the +Pr polarization state (corresponding to the upper portion of the hysteresis loop) when a positive (+) voltage is applied to the upper electrode of the capacitor with respect to the lower electrode, as shown in FIG. 2. On the contrary, the ferroelectric material is in the -Pr polarization state (corresponding to the lower portion of the hysteresis loop) when a positive (+) voltage is applied to the lower electrode of the capacitor with respect to the upper electrode, as shown in FIG. 3.
By changing the polarity of a voltage at both ends of the capacitor, a write information is accomplished in the ferroelectric capacitor so that its polarization state is the same as one of two polarization states shown in the hysteresis loop of FIG. 1. Also, a read pulse must be applied to the lower electrode of the capacitor in order to discriminate the polarization state (i.e. the information) written in the ferroelectric capacitor.
The reading process is as follows.
First, when a read pulse having the same polarity as that of a write pulse voltage is applied to the lower electrode of the ferroelectric capacitor, the polarization state is maintained without polarization switching by the read pulse, from -Pr to -Ps along the lower portion of the hysteresis loop as shown in FIG. 4. Here, the change of the polarization state along the hysteresis loop appears as an electrical charge amount at both ends of the electrode.
On the contrary, when a read pulse having the opposite polarity to that of a write pulse is applied to the lower electrode of the ferroelectric material, the polarization state is changed through a polarization switching by the read pulse, from -Pr to +Pr along the hysteresis loop as shown in FIG. 5. The polarization state change by the switching (i.e. the change of polarity) corresponds to a destructive readout where recorded information is lost by the change of polarity. Thus, in the case of the destructive readout causing the change of polarity, a writeback process is required to turn back the polarization state to the initial state. If the writeback process is not completed, the polarization state is not recovered, and the information is lost. The writeback process is essential for the operation of the FRAM device so long as its circuit structure or reading method is not changed. Due to the polarity inversion caused when the polarity of the pulse applied for writing and that of the pulse applied for reading are different, an FRAM device in which two pulses are applied to a plate line has been developed by Ramtron Co. That is, according to the structure of the circuit developed by Ramtron Co., the first pulse as a read pulse determines the polarization state of the ferroelectric material and the second pulse as a writeback pulse restores the polarization state of data to the state before the reading process. For example, the first pulse of a signal P1 in FIG. 8 is for reading and the second pulse thereof is for the writeback. However, the ferroelectric memory device having the writeback function does not include a safety device for power failures. Accordingly, when the power supply is interrupted during the writeback, the polarization state cannot be completely recovered to the initial state, so that a destructive readout is performed resulting in a loss of data. That this error cannot be recovered is a fatal defect in the memory device. That is, the error in the writeback which is caused by the interrupt of the power supply takes non-volatility away from the FRAM, wherein non-volatility of the FRAM enables data to be kept during a power failure. Even though the probability that a power failure will occur during the writeback is low, the error in the writeback may be a fatal defect in the reliability of the product.